1. Field of the Invention
The present invention relates to an image formation device such as a digital copying machine, computer printer, or network printer, and an image processing device for such an image formation device.
2. Description of the Related Art
In most of the image formation devices currently available on the market, such as a digital copying machine, computer printer, or network printer, the electrophotographic system that allows acquisition of a high quality image with a high speed is widely adopted as an image output device.
In the electrophotographic system, the insulated toner and magnetic particles are mixed and rubbed in a developing roller to thereby electrify the insulated toner, the developing agent is formed in a brush on the developing roller by the magnetic force, and the developing agent is supplied onto the photosensitive structure by the rotation of the developing roller, whereby the static latent image is developed on the photosensitive structure. This method is called the two-component magnetic brush developing system, which is widely used in various fields and widely adopted especially in the color image formation device.
However, in the image output unit of this electrophotographic system, specially in the image output unit based on the two-component magnetic brush developing system, the density of the rear end area of the halftone area which borders on the background area decreases due to a non-linear and asymmetric output characteristic of the image output unit, when an outputted image varies from the halftone area to the background area in the sub-scanning direction.
That is, as shown in FIG. 21A, when the o utputted image varies from a halftone area 1 to a background area 2 in the sub-scanning direction being the reverse direction to the paper feeding direction, which is perpendicular to the main scanning direction of the laser beam for forming the static latent image on the photosensitive structure, the density of a rear end area 1B of the halftone area 1 which borders on the background area 2 decreases on the reasons as shown hereunder.
As shown in FIG. 22, in the electrophotographic system using the two-component magnetic brush developing system, during the rotation of a photosensitive drum 310 along the direction of an arrow 311, an electrifier 320 for forming the static latent image electrifies the photosensitive drum 310, and the laser beam L modulated by an image signal irradiates the electrified photosensitive drum 310, whereby the photosensitive drum 310 forms the static latent image thereon. The photosensitive drum 310 having the static latent image formed thereon comes in contact with a developing agent layer 337 on the surface of a developing sleeve 335 that rotates in the direction of an arrow 336 at about twice the linear velocity against that of the photosensitive drum 310. Thereby, the toner in the developing agent layer 337 is affixed to a latent image portion on the photosensitive drum 310, thereby developing the static latent image on the photosensitive drum 310 into the toner image.
FIG. 22A illustrates a state at the moment when the irradiation of the laser beam L forms a latent image portion 3 corresponding to the halftone area 1 on the photosensitive drum 310, and a forward edge 3f of the latent image portion 3 comes in contract with the developing agent layer 337. FIG. 22B illustrates a state at the moment when a part slightly before a backward edge 3b of the latent image portion 3 comes in contract with the developing agent layer 337. FIG. 22C illustrates a state at the moment when the backward edge 3b of the latent image portion 3 comes in contract with the developing agent layer 337.
The developing sleeve 335 is given the developing bias of, for example, xe2x88x92500 V. The photosensitive drum 310 is electrified at the potential of, for example, xe2x88x92650 V by the electrifier 320. The latent image portion 3 corresponding to the halftone area 1 is electrified at, for example, xe2x88x92200 V which is lower than the developing bias potential. The electrified potential of a part 4 corresponding to the background area 2 at the back of the halftone area 1 becomes xe2x88x92650 V which is higher than the developing bias.
As shown in FIG. 22A, when the forward edge 3f of the latent image portion 3 comes in contact with the developing agent layer 337, a forward electric field for the developing bias is applied to toner tq lying at a position Q where the photosensitive drum 310 comes in contact with the developing agent layer 337. The toner tq is pulled toward the surface of the developing agent layer 337, and is affixed on the latent image portion 3. However, as shown in FIG. 22B, when the part 4 corresponding to the background area 2 at the back of the halftone area 1 comes close to the developing agent layer 337, toner tb lying at a part of the developing agent layer 337 which faces the part 4 is driven away from the surface of the developing agent layer 337, and dives deep into the developing agent layer 337.
While the developing sleeve 335 rotates in the direction of the arrow 336, the toner tb approaches to the position Q where the photosensitive drum 310 comes in contact with the developing agent layer 337, and moves toward the surface of the developing agent layer 337 due to the lower potential of the latent image portion 3. However in this movement, there occurs a time delay for the toner to reach the surface of the developing agent layer 337. Therefore, as shown in FIG. 22B, the amount of the toner affixed on the photosensitive drum 310 decreases from the moment when the part slightly before the backward edge 3b of the latent image portion 3 comes in contract with the developing agent layer 337, and as mentioned above decreases the density of the rear end area 1B of the halftone area 1 which borders on the background area 2.
When the front of the halftone area 1 is also a background area and the forward edge 3f of the latent image portion 3 also comes in contact with the developing agent layer 337, as shown by toner tf in FIG. 22A, some toner is driven away from the surface of the developing agent layer 337 by a part 5 on the photosensitive drum 310 corresponding to the front of the background area.
However, accompanied with the rotation of the developing sleeve 335 in the direction of the arrow 336, the toner tf quickly moves away from the position Q where the photosensitive drum 310 comes in contact with the developing agent layer 337. And at the same time, the toner tq pulled close to the surface of the developing agent layer 337 by the lower potential of the latent image portion 3 approaches the position Q immediately, and is affixed on the latent image portion 3. Therefore, if the outputted image varies from the background area to the halftone area 1 in reverse to the sub-scanning direction, the density of the front end area of the halftone area 1 which borders on the background area does not decrease.
Thus, in the electrophotographic system using the two-component magnetic brush developing system, when the outputted image varies from the halftone area 1 to the background area 2 in the sub-scanning direction, the density of the rear end area 1B of the halftone area 1 which borders on the background area 2 decreases due to the partial reduction from the average of the toner density on the surface of the developing agent layer 337 overlying the developing sleeve 335. In this specification, this density reduction is called TED (Trail Edge Deletion).
This TED can be reduced to some extent by approximating the linear velocity of the developing sleeve 335 to that of the photosensitive drum 310. However, even if the linear velocity of the developing sleeve 335 is made equal to that of the photosensitive drum 310, it is difficult to completely annul TED and develop the toner with ample quantity.
Accordingly, in the Japanese Published Unexamined Patent Application No. Hei 5-281790 and No. Hei 6-87234, there is disclosed a concept that prevents the density reduction as the foregoing TED by enhancing the accuracy of the laser beam scanner that writes a static latent image on a photosensitive structure by a laser beam, and by adjusting the parameters of the developing part that develops the static latent image to thereby enhance the contrast of the developing electric field.
However, the method of enhancing the contrast of the developing electric field by enhancing the accuracy of the laser beam scanner being a part that writes a static latent image tends to invite the size expansion and cost increase of the image output unit. Moreover, if the screen line number is increased for a higher resolution of an output image by the image output unit, the contrast of the developing electric field is lowered, and the density reduction as TED is likely to occur, which makes it difficult to achieve both the elimination of the density reduction and the enhancement of the resolution of the output image.
In recent years, accompanied with the widespread of computer printers and network printers, opportunities are increasing for printing graphic images created on a host computer such as a personal computer and the like. In such graphic images, the density reduction as TED is more obvious than natural images such as photographs. Therefore, in the image formation device such as the computer printer and the network printer, etc., the density reduction as TED develops into a greater problem than in the image formation device of the copying machine, etc.
As a method for correcting a linear and symmetrical output characteristic of an image output unit, such as the MTF characteristic, the system that corrects input image data by the digital filtering processing is widely accepted. However, since there is a narrow domain for the digital filtering processing, it is impossible to decrease or prevent the density reduction as TED that occurs over a wide range in the sub-scanning direction based on a nonlinear and asymmetrical output characteristic of an image output unit as mentioned above.
Accordingly, the applicant disclosed a proposal to prevent the density reduction as TED by correcting image data in the Japanese Published Unexamined Patent Application No. Hei 10-65917.
Since it corrects image data value of a halftone area linearly on the basis of the image data value of an edge in which an outputted image varies from the halftone area to the background area in the sub-scanning direction, this method makes it possible to easily achieve enhancement of the resolution of the output image without inviting the size expansion and cost increase of the device, and to prevent the density reduction as TED.
Further, in the Japanese Published Unexamined Patent Application No. Hei 10-65920, the applicant proposed to enhance the correction accuracy of TED by describing the characteristic of a density variation into a characteristic description part, and correcting the described characteristic on the basis of the outputted image in the image output unit. Furthermore, in the Japanese Published Unexamined Patent Application No. Hei 11-32214, the applicant proposed to enhance the correction accuracy of TED with the image data value after the edge taken into consideration.
However in the methods of these applications, since the correction value for the image data of the halftone area is calculated on the basis of the image data value at the edge and after the edge, and further in accordance with the position from the edge, the correction processing will become considerably large in scale.
Accordingly, it is an object of the present invention to prevent the density reduction in the rear end area of the halftone area which borders on the background area, when the outputted image varies from the halftone area to the background area in the sub-scanning direction, by simplified and small-scale processing without inviting the size enlargement and cost increase of the image formation device or the image output device.
In order to accomplish the object of the present invention, the image formation device that forms a color image on a recording medium is comprised of an edge extraction part that extracts an edge in which multivalued image data of each recorded color varies from halftone image data to background image data in the sub-scanning direction on the recording medium, and an image data correction part that modifies a value of the background image data close to the edge extracted by the edge extraction part, of the multivalued image data, to a setting value close to an image recurrence start point of the image formation device.
In this case, the value of the background image data in a range having specific m pixels in the sub-scanning direction from the edge, or in a range having specific m pixels in the sub-scanning direction from the edge and specific n pixels in the main scanning direction, or in a range having specific m pixels in the sub-scanning direction with the edge on the center and specific n pixels in the main scanning direction can be modified to the setting value. Further in this case, the setting value can be made to be designated by a user.
In the image formation device of the present invention thus configured, the value of the background image data close to an edge in which the multivalued image data varies from the halftone image data to the background image data in the subscanning direction is modified to the setting value, namely, a value close to an image recurrence start point of the image formation device. Therefore, as shown in FIG. 21A, when the outputted image varies from the halftone area 1 to the background area 2 in the sub-scanning direction, a latent image is formed to a non-developing degree on the part 4 corresponding to the background area 2 on the photosensitive drum 310 shown in FIG. 22, and the potential of the part 4 approximates to the developing bias potential applied to the developing sleeve 335.
Accordingly, the reverse electric field applied to the toner contained in the developing agent layer 337 becomes extremely weak, and the toner contained in the developing agent layer 337 will not be driven away from the surface of the developing agent layer 337. Accordingly, when the rear end part of the latent image portion 3 on the photosensitive drum 310, corresponding to the rear end area of the halftone area 1 that borders on the background area 2, comes in contact with the developing agent layer 337, the toner is affixed on the latent image portion 3 of the photosensitive drum 310 without a time delay, by the forward developing electric field due to the lower potential of the latent image portion 3.
Therefore, the density reduction of the rear end area of the halftone area 1 that borders on the background area 2 can be prevented. And in the present invention, to prevent the density reduction is only needed to replace the value of the background image data close to the edge by the setting value, for which suffices simplified and small-scale processing.
Similarly, in the image formation devices of the present invention as claimed in claims 2, 3, 4, 7, 8, or in the image processing devices of the present invention as claimed in claims 9, 10, 11, 12, 13, 16, 17, 18, the modification of the value of the background image data close to the edge, or all the values of the background image data to the setting value makes it possible to prevent the density reduction of the rear end area of the halftone area that borders on the background area.